In wireless security systems, power conservation in remote devices, such as keypads or key fobs, is an important requirement since they are typically battery powered. Recently, some remote devices have incorporated a speaker/microphone system so that the user can communicate with an operator at the central control station during an alarm situation. These remote device speakers can have audio quality similar to a public address system and therefore can use an amplifier that sacrifices fidelity for greater power conservation.
Class D amplifiers are typically used for speakers where power conservation is a requirement. The class D amplifiers use an H bridge driver circuit that has four switching elements each connected across the speaker. A pulse width modulation (PWM) audio signal drives two switches at a time, one that pushes the speaker and the other that pulls the speaker back to its origin (side A). When the PWM input signal changes polarity, the opposite two switches become operational with one switch pushing the speaker (in the direction opposite of side A) and the other switch pulling the speaker back to its origin (this is side B). Because the signals are comprised of pulses that have fixed amplitudes (but varying widths), the switching elements (usually MOSFETs) are switched either on or off rather than operated in their linear mode. This means that the only power dissipated by the transistors is during the interval between the on and off states. The class D amplifiers use very high switching frequencies that are typically greater than 100 KHz to achieve good fidelity with low distortion.
In the security industry there is always a need to further decrease the power consumption of the products and to further reduce the costs of the products. The present invention is an improved class D amplifier that accomplishes both these tasks. It also decreases the emission of electro-magnetic interference (EMI), which is an additional concern when designing security products.
It is therefore an object of the present invention to provide a class D amplifier with increased efficiency while providing acceptable sound quality for a speaker in a remote security device.